Liquefaction methods and systems where refrigeration is generated by expanding gaseous refrigerant in a reverse-Brayton cycle are known. These methods and systems typically employ two expanders where the gaseous refrigerant is expanded to substantially the same pressure within tolerance of the pressure drop through equipment. Some systems also include more than two expanders with the cold expander discharge pressure being higher than the discharge pressures of the remaining expanders. These methods and systems have potentially simple compression systems because there are no streams introduced between compression stages, and simple heat exchangers because there are less passages and headers. Further some methods and systems use an open-loop system that utilizes the liquefied fluid as a refrigerant.
The previous methods and systems for liquefaction, however, are problematic for several reasons. For example, using simple compression systems and simple heat exchangers fails to result in improved efficiencies. Moreover, the cost savings in using an open-loop system does not outweigh the flexibility of using a closed-loop system.
There is a need for a method and system for liquefaction where the steps of precooling, liquefaction, and subcooling are more safe, efficient, and reliable.